MDEV-24242 Query returns wrong result while using big_tables=1

When executing set operations in a pipeline using only one temporary table
additional scans of intermediate results may be needed. The scans are
performed with usage of the rnd_next() handler function that might
leave record buffers used for the temporary table not in a state that
is good for following writes into the table. For example it happens for
aria engine when the last call of rnd_next() encounters only deleted
records. Thus a cleanup of record buffers is needed after each such scan
of the temporary table.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index 021720f594e..9e51bb43a74 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -880,6 +880,10 @@ bool select_unit_ext::send_eof()
     table->file->ha_rnd_end();
   }
 
+  /* Clean up table buffers for the next set operation from pipeline */
+  if (next_sl)
+    restore_record(table,s->default_values);
+
   if (unlikely(error))
     table->file->print_error(error, MYF(0));